Depression is a mental state characterised by persistent low mood, loss of interest and enjoyment in everyday activities, neurovegetative disturbance, and reduced energy, causing varying levels of social and occupational dysfunction. Depressive disorders are common, and they are the fourth highest cause of disability worldwide.   In people aged 18 to 44 years, depression is the leading cause of disability and premature death, and it is predicted to be the second leading cause of disability in people of all ages by the year 2020. 
Depressive disorders may be divided into minor depression, dysthymia, and major depressive disorder, depending on the length and number of symptoms, the degree of functional impairment, and the severity of symptoms.   In bipolar disorder, major depressive disorder is accompanied by or interspersed with 1 or more manic or mixed episodes.